Microwave waveguide waterload employing a quarter wave window transformer



Oct. 21, 1969 z RL 3,474,360

MICROWAVE WAVEGUIDE WATERLOAD EMPLOYING A QUARTER WAVE WINDOWTRANSFORMER Filed June 14, 1968 to 1'4 is, 812 8:6

FREQ EN (6H2) RICHARD z. GERLACK FIG. 5 p W ATTORNEY INVENTOR.

United States Patent MICROWAVE WAVEGUIDE WATERLOAD EM- PLOYING A QUARTERWAVE WINDOW TRANSFORMER Richard Z. Gerlack, Cupertino, Calif., assignorto 'Varian Associates, Palo Alto, Calif., a corporation of CaliforniaFiled June 14, 1968, Ser. No. 737,134 Int. Cl. H01p 1/26 US. Cl. 333-227 Claims ABSTRACT OF THE DISCLOSURE A microwave waveguide load isdisclosed. The load includes a section of hollow waveguide adapted to beconnected at one end to the source of microwave energy to be absorbed.The other end of the section of waveguide looks into a stream ofdielectric lossy coolant, such as water, contained within a dielectrictube extending across the end of the section of waveguide to form aload. The wall thickness of the dielectric tube is made an integralnumber of quarter wavelengths thick to form a waveguide impedancetransformer between the impedance of the waveguide and the impedance ofthe coolant stream, whereby a broadband match is obtained to thedielectric coolant stream forming the load. The tubular dielectricwindow is sealed to the end of the waveguide by 0 rings, in oneinstance, or by brazing to an annular frame member, in a secondinstance. The tubular construction of the window facilitates fabricationof the load and reduces the manufacturing costs thereof.

DESCRIPTION OF THE PRIOR ART Heretofore, microwave waveguide waterloadshave been constructed which employ quarter wave window transformers formatching the impedance of the waveguide to the impedance of the coolantstream to obtain a wide passband high power load. However, in the priorart, the window transformer sections have comprised fiat blocks ofdielectric, such as alumina ceramic or Tefion. In the case of a ceramicwindow member it is typically a ceramic block brazed into the end of thewaveguide section which looks into the coolant stream. The problem withthe use of a flat block type dielectric window, as sealed into the endof a section of waveguide, is that the window must be sealed around itsperiphery, i.e., on all four sides of the block window typically bybrazing a metallized ceramic block into a thin sheet metal frame as ofcopper, which in turn is brazed into the waveguide. This type of windowconstruction is relatively expensive. Such a prior art waveguidewaterload is disclosed and claimed in US. Patent 3,360,750, issued Dec.26, 1967, and assigned to the same assignee as the present invention.Therefore, a need exists for a transformer window construction which ismore easily sealed to the waveguide, to facilitate fabrication of thewaveguide waterload.

SUMMARY OF THE PRESENT INVENTION The principal object of the presentinvention is the provision of an improved microwave waveguide waterloademploying a quarter wave window transformer.

One feature of the present invention is the provision, in a microwavewaveguide waterload of the type employing a quarter wave windowtransformer, of a dielectric tube for containing the stream of lossydielectric coolant, such tube having a wall thickness of approximatelyan integral number of quarter wavelengths to form the windowtransformer, whereby fabrication of the window portion of the load isfacilitated.

Another feature of the present invention is the same as the precedingfeature wherein the tubular dielectric window member includes a sectionhaving axially spaced end portions which are sealed to opposite sidewalls of the waveguide section.

Another feature of the present invention is the same as any one or moreof the preceding features wherein a pair of resilient 0 rings aredisposed at opposite ends of the tubular dielectric window for sealingthe window to the waveguide.

Other features and advantages of the present invention become apparentupon a perusal of the following specification taken in connection withthe accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectionalview of a waveguide waterload employing features of the presentinvention,

FIG. 2 is a sectional View of a portion of the structure of FIG. 1 takenalong line 22 in the direction of the arrows,

FIG. 3 is a longitudinal sectional view of an alternative waterloadincorporating features of the present invention,

FIG. 4 is a sectional view of the structure of FIG. 3 taken along lines44 in the direction of the arrows, and

FIG. 5 is a plot of voltage standing wave ratio versus frequency, ingigahertz, depicting the passband characteristics of the waterload ofthe present invention as a function of the temperature of the water.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1 and 2,there is shown a Waveguide waterload 1 incorporating features of thepresent invention. The waveguide waterload 1 includes a first section ofrectangular waveguide 2 joined, as by brazing or welding, at 3 to acylindrical section of waveguide 4. The cylindrical section of waveguide4 has its longitudinal axis parallel to the broad walls 5 of therectangular waveguide section 2. A flange 6 is aflixed to the other endof the waveguide section 2 for connecting the waveguide waterload 1 to asource of microwave energy to be absorbed.

A hollow cylindrical tubular dielectric window member 7, as of aluminaceramic, is coaxially disposed within the cylindrical waveguide section4. The outside diameter of the tubular window member 7 is substantiallythe same as the inside diameter of the cylindrical waveguide section 4to prevent the formation of an air gap in the region of the strongelectric fields at the junction 3 of the rectangular waveguide section 2with the cylindrical waveguide section 4. The tubular window 7 has awall thickness which is approximately an integral number of quarterelectrical wavelengths thick at a frequency corresponding to the centerof the passband of the waterload. An electrical wavelength is defined asthat distance in the direction of power flow of wave energy in atransmission line through which the wave energy experiences 360 of phaseshift.

A stream of lossy dielectric liquid coolant 8, as indicated by thearrows 8, is directed through the hollow interior of the tubular window7 for absorbing microwave energy passing through the wall of the tubularwindow 7 from the waveguide section 2. For the case where the lossydielectric liquid coolant stream 8 is water having a dielectric constantof approximately the window member 7 is preferably made of aluminaceramic and is preferably an odd number of quarter electricalwavelengths thick to provide a relatively wide band quarter wavetransformer section between the section of rectangular waveguide 2 andthe impedance of the lossy dielectric coolant stream 8. In case thelossy dielectric coolant is ethylene glycol or a mixture of ethyleneglycol and water having a substantially lower dielectric constant thanwater, the tubular window member 7 preferably has a lower dielectricconstant, such as that provided by Teflon (polytetrafiuoroethyleneresin). In case a tubular Teflon window member 7 is employed, the windowmember may advantageously be made an even number of quarter electricalwavelengths thick to provide a resonant half-wave window between thestream of lossy dielectric coolant 8 and the waveguide section 2.

The tubular window member 7 is sealed to the waveguide sections 4 and 2by means of a pair of resilient rings 9, as of rubber, neoprene orhollow metallic tubing, to provide a liquid impervious barrier acrossthe end of the rectangular waveguide section 2. The 0 rings 9 arecaptured between the end of the tubular window member 7 and a pair ofcentrally apertured cover plates 11 carried from the ends of the tubularwaveguide 4 via a plurality of screws -12. Threaded pipe nipples 13 areprovided in the center of each of the cover plates 11 for connecting thetubular waveguide 4 to coolant pipes, not shown, for supplying thecoolant and extracting the coolant from the waterload 1.

Typically, the quarter wave transformer window member 7 does not providea perfect impedance match over a relatively wide passband between thewaveguide 2 and the stream 8 of lossy dielectric coolant. Therefore, animpedance matching wave reflective discontinuity 14, such as aconductive capacitive septum, is provided in the waveguide section 2 aproper distance from the adjacent face of the window 7 to produce a wavereflection of the proper magnitude and phase to cancel the wavereflection from the mismatch between the window member 7 and the streamof lossy dielectric coolant 8. In the case of a capacitive conductiveseptum 14 which extends across the rectangular waveguide section 2 fromone narrow wall 15 to the opposed narrow wall 15 and parallel to thebroad walls 5, the septum 14 is preferably placed a quarter electricalwavelength from the adjacent face of the tubular window 7.

A waveguide waterload 1 of the type described in FIGS. 1 and 2 employingan alumina ceramic window 7 and water as the lossy dielectric coolant 8,provides a passband characteristic of the type shown in FIG. 5, wherevoltage standing wave ratio is plotted versus frequency for threedifferent temperatures of the water. From the plot of FIG. it is seenthat the waveguide load 1 has a relatively wide passband between voltagestanding wave ratios points of 1.10.

One advantage of the waveguide waterload 1 of FIGS. 1 and 2 is that thetubular dielectric window member 7 is sealed into the waveguidestructure '2 and 4 by means of a pair of conventional 0 rings 9, therebyeliminating the necessity of a brazed joint between the waveguide 2 or 4and the window member 7, whereby fabrication of the waterload 1 isfacilitated and the manufacturing cost thereof reduced.

As an alternative to having the longitudinal axis of the cylindricalwaveguide 4 parallel to the broad walls 5 of the waveguide section 2,the longitudinal axis of the cylindrical waveguide section 4 may beparallel to the narrow walls 15 of the rectangular guide 2, as indicatedby the phantom lines 5 and 15 in FIG. 2. In this latter case, the 0rings 9 will be located in a region of more intense electric fields thanin the first embodiment, and in such a case it is preferred to employmetallic resilient 0 rings to prevent over-heating thereof. In bothcases where 0 rings 9 are employed for sealing the tubular window 7 tothe waveguide sections 2 and 4, it is preferred that the tubular window7 have a length substantially longer than either the breadth or heightof the waveguide section 2 to remove the resilient 0 rings 9 from theimmediate vicinity of the strong electric fields produced at thejunction 3 of the rectangular waveguide 2 with the cylindrical waveguide4.

Referring now to FIGS. 3 and 4, there is shown an alternative embodimentof the waterload 1 of the present invention. The embodiment of FIGS. 3and 4 is similar to the latter described alternative embodiment towaterload -1 of FIGS. 1 and 2 wherein the longitudinal axis of thecylindrical waveguide section 4 is parallel to the narrow side walls 15of the rectangular waveguide section 2. More specifically, in theembodiment of FIGS. 3 and 4, a relatively short length of cylindricalwaveguide section 4 is formed in a pair of conductive metallic blocks 21which are in turn brazed into a circular bore 22 in the top and bottomwalls 5 of the rectangular waveguide section 2. A pair of annular sheetmetal frame members 23, as of copper, are brazed to the ends of thetubular window member 7 and to the top and bottom walls 5 of thewaveguide 2, which are .defined by the innerfaces of the conductiveblocks 21. A conductive plate 22 closes off the end of the rectangularwaveguide 2. A pair of coolant passageways 25 are provided by bores, inblocks 21, which intersect with the closed ends of the cylindricalwaceguide sections 4. The longitudinal axes of the passageways 25 aredirected at that inside face portion of the tubular window 7, which isin registration with the center of the rectangular waveguide section 2,such that the microwave power which passes through the tubular window 7is rapidly carried away from the window 7 for enhanced cooling andabsorption of microwave power.

Although the waterload embodiment of FIGS. 3 and 4 employs a brazedwindow construction which is more expensive to manufacture than the Oring seals of the embodiments of FIGS. 1 and 2, the brazed assembly ofFIGS. 3 and 4 is more easily fabricated than the prior art rectangularwindows because the seals need only be provided at the axial ends of thewindow member 7, whereas the prior art rectangular window required thata seal be made entirely around the four sides of the window member. Thewaterload 1 of FIGS. 3 and 4 is especially useful with reduced heightwaveguide 2 as a resistive load termination for a severed microwaveslowwave tube circuit.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontainer in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a microwave waveguide load, means forming a hollow waveguide,means forming a microwave permeable dielectric window member sealedacross said waveguide to form a liquid impervious barrier across one endof a section of said waveguide, means for connecting said waveguide to asource of microwave energy to be absorbed, means for passing a stream oflossy dielectric liquid coolant adjacent a face of said window memberwhich is on the opposite side of said barrier from the source of energyto be absorbed for absorbing microwave energy passing into the stream ofliquid coolant from the source of microwave energy, said window memberhaving a thickness taken in the direction of power flow therethroughinto the liquid stream which is approximately an integral number ofquarter wavelengths at the center frequency of the passband of the loadfor matching the impedance of said section of waveguide to the impedanceof the lossy liquid coolant, the improvement wherein said dielectricwindow member is a tube of dielectric material for containing the streamof lossy dielectric coolant, said dielectric tube having a wallthickness of approximately the integral number of quarter electricalwavelengths at the center frequency of the passband of the load forimpedance matching, whereby fabrication of the window portion of theload is facilitated.

2. The apparatus of claim 1 wherein said section of waveguide is hollowrectangular waveguide having broad top and bottom walls and a pair ofnarrow side walls, and wherein the longitudinal axis of said dielectrictube is parallel to the broad walls of said waveguide, and means forminga sealing structure sealing the end of at least a portion of saiddielectric tube to said waveguide section.

3. The apparatus of claim 2 wherein said sealing structure includes apair of resilient 0 rings disposed at opposite ends of said dielectrictube portion.

4. The apparatus of claim 1 wherein said section of waveguide is hollowrectangular waveguide having a broad top and bottom wall and a pair ofnarrow side Walls, and wherein the longitudinal axis of said dielectric10 tube is parallel to the narrow walls of said waveguide, and meansforming a sealing structure sealing the ends of at least a portion ofthe length of said dielectric tube to said waveguide section.

5. The apparatus of claim 1 wherein said dielectric tube is made ofceramic and the dielectric liquid is predominately water.

6. The apparatus of claim 1 wherein the wall thickness of said tubulardielectric window member is approximately one quarter electricalwavelength.

7. The apparatus of claim 5 wherein said dielectric tube is made ofpolytetrafluoroethylene resin and the dielectric constant of the liquidcoolant is substantially less than 80.

References Cited UNITED STATES PATENTS 3,289,109 11/1966 Nelson 33322FOREIGN PATENTS 669,250 4/ 1952 Great Britain.

HERMAN KARL SAALBACH, Primary Examiner 15 M. NUSSBAUM, AssistantExaminer US. Cl. X.R. 333-98, 98F

